This invention relates to an apparatus and method for monitoring the presence of a flame.
It is known to use a monitor with radiation sensing means for the remote detection of a flame. The direct illumination from the flame, characterised by its flickering nature, is distinguished from the sensed radiation by filtering out of the signal any steady-state background illumination.
Further precautions may be needed where there may be multiple sources of fluctuating radiation, such as exist in multi-burner equipment, to ensure that the radiation from another source does not influence the reading for the flame being monitored. In industrial boiler installations, for example, there may be a bank of closely spaced burners each of which have to be monitored individually. The space available may then be so limited that it is not possible to site each radiation detector where its line of sight will impinge on the combustion zone of only a single burner. Typically, the detector may be located to one side of the burner with its optical axis inclined towards the burner axis so as to enter the flame at a point along its length nearer the burner outlet than the tip of the flame. If the flame should be extinguished, the combustion zone of another flame could become visible along the line of sight. This problem is further complicated by the fact that the loss of a flame from one burner may allow the flames of adjacent burners to spread towards the space previously occupied by the extinguished flame.
A solution to this problem is offered by an apparatus known from GB 1396384 in which there are two radiation sensors directed onto the flame from one side of the burner, as already mentioned, but aligned on axes that intersect near their point of entry into the flame. Signal processing means for the output signals from the sensor devices filter out any non-identical components from the two signals to give the processed signal that, in principle, is dependent on the fluctuating radiation from the zone of intersection of the two detector axes.
Such an apparatus, while it offers an effective solution for the problem, is inherently both expensive and space-consuming because it requires the two radiation sensors and the joint processing of their outputs. It is an object of the present invention to provide a more cost-effective approach.